Solidification microstructure in a supercooled binary alloy

The maximum undercooling that has been achieved for Ni-Cu alloy, by using molten glass purification and cyclic super-heating technology, is 270 K. With the help of high-speed photography, the solidification front images of Ni-Cu alloy at various typical undercooling were obtained. Two grain refinements occurred in the range of 60 K< ΔT < 100 K and ΔT > 170 K, the solidification front became smoother, and the solidification position appeared randomly. With the increase of undercooling, the transition from solute diffusion to thermal diffusion leads to the transition from coarse dendrite to directional fine dendrite. At large undercooling, considerable stress is accumulated and some dislocations exist in the microstructure. However, the proportion of high-angle grain boundaries is as high as 89%, with twin boundaries of 13.6% and most strain-free structures, and the microhardness decreases sharply. This indicates that the accumulated stress at large undercooling causes the plastic strains in the microstructure, and in the later stage of recalescence, part of the plastic strains is dissipated by the system and acts as the driving force to promote the recrystallization of the microstructure.

Modeling the Filler Phase Interaction with Solidification Front in Al(TiC) Composite Produced by the In Situ Method

This paper presents simulation results of the interaction of TiC nanoparticle in liquid aluminum. The behavior of the TiC particle in the frontal interaction region stems from the operation of a system of such forces as gravity, viscous flow drag force, and Saffman force. The difference in density between the TiC and the aluminum matrix makes the particle fall, regardless of the radius dimension; while the Saffman force—which accounts for the local velocity gradient of the liquid aluminum—causes that particles with the smallest radii considered in the calculations 6.4 × 10−8 m; 7 × 10−8 m; 7.75 × 10−8 m; 9.85 × 10−8 m are repelled from the solidification front and the particles with 15.03 × 10−8 m are attracted to it. The viscosity growth in the course of casting caused by the lowering temperature reduces this effect, though the trend is maintained. The degree to which the particle is attracted to the front clearly depends on the velocity gradient of the liquid phase. For a very small gradient of 0.00001 m/s, the particle is at its closest position relative to the front.

Influence of the temperature field in the energy-saving carbon-graphite thermal unit on configuration of the solidification front under different conditions of growing fluoride single crystals by the HDS method

Mathematical modeling of the influence of the temperature field created by the ECU on the position and on configuration of the solidification front under different conditions of growing single crystals of fluorides by the HDS method has been carried out. Data were obtained for different growing conditions for single crystals of fluorides and their effect on configuration of the solidification front in various modifications of the ECU of the first and second types. From a comparison of mathematical calculations, a conclusion was made about the most suitable conditions for the synthesis of these single crystals.

Inverse Columnar-Equiaxed Transition (CET) in 304 and 316L Stainless Steels Melt by Electron Beam for Additive Manufacturing (AM)

According to Hunt’s columnar-to-equiaxed transition (CET) criterion, which is generally accepted, a high-temperature gradient (G) in the solidification front is preferable to a low G for forming columnar grains. Here, we report the opposite tendency found in the solidification microstructure of stainless steels partially melted by scanning electron beam for powder bed fusion (PBF)-type additive manufacturing. Equiaxed grains were observed more frequently in the region of high G rather than in the region of low G, contrary to the trend of the CET criterion. Computational thermal-fluid dynamics (CtFD) simulation has revealed that the fluid velocity is significantly higher in the case of smaller melt regions. The G on the solidification front of a small melt pool tends to be high, but at the same, the temperature gradient along the melt pool surface also tends to be high. The high melt surface temperature gradient can enhance Marangoni flow, which can apparently reverse the trend of equiaxed grain formation.

The Influence of the Cooling Bores on Crystal Orientation and Lattice Parameter in Single-Crystalline Cored Turbine Blades

The areas located near the cooling bores of single-crystalline cored turbine blades made of nickel-based CMSX-4 superalloy were studied. The blades were solidified by the vertical Bridgman technique in the industrial ALD furnace. Longitudinal sections of the blades were studied by Scanning Electron Microscopy, X-ray diffraction topography, X-ray diffraction measurements of the γ′-phase lattice parameter a, and the α angle of the primary crystal orientation. The local changes in α were analyzed in relation to the changes of the dendrite’s growth direction near the cooling bores. It was found that in the area approximately 3 ÷ 4 mm wide around the cooling bores, changes of α and a, both in the blade root and in the airfoil occurred. The local temperature distribution near the cooling bores formed a curved macroscopic solidification front, which caused changes in the chemical composition and, consequently, changes in the a value in a range of 0.002 Å to 0.014 Å. The mechanism of alloying elements segregation by tips of the dendrites on the bent solidification front was proposed. The multi-scale analysis that allows determining a relation between processes proceed both on a millimeter-scale and a micrometric and nanometric scale, was applied in the studies.

Shape Deviation of Surface Structures Produced by WaveShape (Structuring by Laser Remelting) on Ti6Al4V and a Method for Deviation Reduction

Laser structuring by remelting (WaveShape) is a manufacturing process for metal surfaces in which structures are generated without material removal. The structuring principle is based on the controlled motion of the three-phase line in the area of the solidification front. The contour of the solidification front is imprinted into the remelting track during the continuous solidification process. Typically, harmonic surface structures in the form of sinusoidal oscillations are generated by means of WaveShape with virtually no material loss. However, a significant shape deviation is often observed over a wide range of process parameters. In this study, it was found that much of the shape deviation is concentrated at a spatial wavelength equal to half the spatial wavelength used for structuring. Therefore, an approach to reduce the shape deviations was specifically investigated by superimposing a compensation signal on the harmonic structuring signal. In this approach, a compensation signal with half the spatial wavelength was varied in phase and amplitude and superimposed on the structuring signal. Amplitude and phase shift of the compensation signal were further investigated for selected laser beam diameters and spatial wavelengths. This demonstrated that a shape deviation of harmonic surface structures on titanium alloy Ti6Al4V could be reduced by up to 91% by means of an adapted compensation signal.

Multiple objects interacting with a solidification front

The interaction of objects suspended in a liquid melt with an advancing solidification front is of special interest in nature and engineering sciences. The front can either engulf the object into the growing crystal or repel it. Therefore, the object-front confrontation can have a strong influence on the microstructure and mechanical or functional properties of the solidified material. The past theoretical models and experimental studies have mostly investigated the interaction of isolated, spherical, and hard objects in pure melts. However, the outcome of object-front interactions in complex (more realistic) systems, where multiple objects and solutes are present, is still poorly understood. Here we show the interaction of multiple oil droplets with an ice-water front in the absence and presence of solute effects using in situ cryo-confocal microscopy. We report on how the object size, number of objects, and bulk solute concentration influence the the object-front interaction and the front morphology, as well as the subsequent object spatial distribution. We suggest that the volume fraction of objects suspended in a liquid melt in conjunction with the amount of bulk solute concentration are two important parameters to be incorporated in the development of object-front interaction models.

In situ observation of the impact of hydrogen bubbles in Al–Cu melt on directional dendritic solidification

Much research has already been focused on the solid-bubble interaction in the interdendritic space for solidifying materials. However, commonly, bubble nucleation is not limited to the mushy zone but also occurs in the liquid melt. In the present research on an Al-$$10 \, \%\mathrm {wt. \,}$$ 10 % wt . Cu alloy, the interaction between these bubbles and the approaching solidification front becomes apparent under in situ X-radiography and allows for new insights into the influence of bubbles on the solidifying microstructure. The observed effects comprise bulging of the solidification front toward the bubble, bending of dendrites in front of the bubble, coronal outgrowths surrounding the bubbles, as well as bubble growth, bubble pushing, and bubble eruption. It is found that for the present Al–Cu alloy, the local variation in the solidification speed can be attributed to the bubbles’ insulating properties. The range of this effect was observed to be up to $$900 \,\upmu \text {m}$$ 900 μ m , depending on the bubble diameter, locally increasing solidification speed by up to $$350 \, \%$$ 350 % . The influences of Marangoni vortices and coronal nucleation of misoriented dendrites around bubbles on the homogeneity of the microstructure are discussed. A comparison with experiments on model alloys and simulations from various other studies highlights the similarities and differences to this metallic alloy system.

Temperature Gradients at the Solidification Front of Deep Hybrid Laser Welds

Grain refinement and the avoidance of columnar solidification is a great challenge in the deep penetration laser welds of thick-section steels. Further knowledge about the heat distribution and the temperature gradients in laser welds is vital for future attempts on the grain refinement of such welds. In this study, a comparative investigation was carried out for full and partial penetration hybrid laser welding of structural steel. The transient temperature distribution and temperature gradients were calculated for the experiments using a simplified three-dimensional finite element analysis. A comparative analysis was presented to investigate the influence of penetration mode on the temperature gradient in the liquid at different weld depths. The results of the numerical analysis suggested that, for given welding process parameters, full penetration welds have lower temperature gradients at the solidification front, meaning that they potentially have a higher chance of grain refinement, compared with partial penetration welds.